The endocrine events occurring near term in the cow are poorly understood especially the alterations in placental function which lead to the peripartal increase in estrogen and prostaglandin synthesis required for normal delivery. Furthermore, the cell types responsible for the synthesis of steroids and prostaglandins in bovine placenta have not been rigorously determined. Preliminary studies have demonstrated the presence of endogenous inhibitors of steroidogenesis and prostaglandin synthesis in bovine placental extracts. The exact role played by these unique factors and steroid hormones in controlling placental functions remains to be explored. The goal of the proposed research is to elucidate the control of endocrine function of the bovine placentome near term and the following specific aims have been formulated: 1) To determine the concentration of steroids and prostaglandins in fetal cotyledon and maternal caruncle during different stages of gestation and to define the pattern of steroid and prostaglandin synthesis by explants of fetal cotyledon and maternal caruncle during 24 hr in vitro incubation. 2) to identify the cell types of the fetal cotyledon and maternal caruncle responsible for steroid and prostaglandin synthesis using enzymatically dispersed placental tissue. 3) To determine the effects of human chorionic gonagotropin and glucocorticoids on steroid and prostaglandin synthesis by the fetal cotyledon and the maternal caruncle and their cell populations. 4) To isolate and characterize two factors in bovine placental tissue, one of which inhibits steroidogenesis and the other prostaglandin synthesis. To quantitate levels of the two inhibitors in placental tissue at different stages of gestation and correlate the levels with plasma and tissue concentrations of steroids and prostaglandins. These studies should further our understanding of regulatory mechanisms involved in parturition in farm animals. The inhibitors isolated from the placental tissue may also prove useful as pharmacological agents affecting reproductive function as well as processes mediated by prostaglandins.